A turbofan gas turbine engine nacelle intake is required to supply the fan of the turbofan gas turbine engine with favourably conditioned air during all operational conditions of the turbofan gas turbine engine, irrespective of the aircraft environment and aircraft attitude, whether the aircraft is in flight or on the ground. The nacelle intake may also be required to absorb noise generated by the gas turbine engine.
Related Art nacelle intakes are designed to minimise the pressure loss of the intake at the maximum incidence the aircraft experiences in flight and at the maximum crosswind conditions on the ground. For example, good pressure recovery at high incidence can be achieved by attempting to ensure that the air flow remains attached to the bottom lip of the nacelle intake by drooping down the nacelle highlight and throat relative to the axis of the fan of the turbofan gas turbine engine.
FIG. 1 is a partially cut away view of a turbofan gas turbine engine having, in axial flow series, an intake 12, a fan section 14, a compressor section 16, a combustion section 18, a turbine section 20 and an exhaust 22. The fan section 14 comprises a fan disc 24 carrying a plurality of circumferentially spaced radially extending fan blades 26. The fan disc 24 and fan blades 26 are surrounded by a fan casing 28. The fan casing 28 is mounted from the core casing 30 by a plurality of radially extending fan outlet guide vanes 32. The fan section 14 is driven by a turbine in the turbine section 20 via a shaft (not shown). The compressor section 16 is driven by a turbine in the turbine section 20 by a shaft (not shown). The whole of the turbofan gas turbine engine 10 is placed within a nacelle 34.
FIGS. 2 and 3 are respectively enlarged vertical and horizontal longitudinal cross-sections, containing the engine axis X, through the intake of the turbofan gas turbine engine nacelle shown in FIG. 1. The nacelle 34 has an intake 12 at its upstream end and an exhaust 25 at its downstream end. The nacelle 34 intake 12 comprises, in flow series, a flared intake lip 36, an intake throat 38 and a diffuser 40 upstream of the fan section 14 of the turbofan gas turbine engine 10. The intake lip 36 forms a contraction for the supply of air to the intake throat 38. The diffuser 40 is arranged to diffuse the air from the intake throat 38 to the fan section 14.
The intake lip has an inner surface 36A and an outer surface 36B. The highlight H is a closed loop running around the intake lip, and defines the boundary between the lip inner and outer surfaces. In FIG. 2 the highlight H is viewed edge on and is indicated by a solid line. In FIG. 3, the part of the highlight H below the plane of the drawing is indicated by a solid line, and the part above the plane of the drawing is indicated by a dashed line.
Generally, the highlight H lies in a highlight surface which is either planar or is curved in only one principal direction. On longitudinal sections through the engine containing the engine axis, the lip inner and outer surfaces are tangency matched at the highlight. Indeed, the lip inner and outer surfaces may be tangential to the highlight surface at the highlight. However, although the curvatures of the inner surface and outer surface are generally at a maximum at the highlight on a longitudinal section, these curvatures may not be the same maximum values. Thus it is typical for the lip to have a discontinuity in curvature across the highlight.
Travelling downstream along the inner surface 36A on a longitudinal section, the end of the inner surface 36A is reached when the tangent to the inner surface becomes 90° relative to the tangent to the inner surface on the same section at the highlight. Generally, on such a section, the curvature of the inner surface 36A decreases continuously in the direction from highlight to the downstream end. The position of the intake throat 38 can be taken to be the downstream end of the inner surface 36A.
A problem with nacelle intakes is that the flow path in the nacelle immediately upstream of the fan may not be symmetric. This can result in an asymmetric flow of air to the fan causing it to operate away from its optimum operating point and hence there is a loss of efficiency. Flow asymmetries may be cause, for example, by crosswinds.
Nacelle intakes may also not be optimised with respect to noise attenuation and other operational considerations.